Recreated Ant build script to match latest Android SDK

Just updated build.xml via command `android update project --path bump-api-library` to get an option to locate SDK with env variable. Was introduced in one of the recent versions of SDK tools.
This could be helpful for using Jenkins to build project depending on the android-websockets library. local.properties file is not mandatory now. Please refer to [similar pull-request](https://github.com/koush/android-websockets/pull/11) for details.